Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 6 defined by servo sectors 40-4N recorded around the circumference of each servo track. Each servo sector 4i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 4i further comprises groups of servo bursts 14 (A,B,C,D in the example shown), which are recorded with precise intervals and offsets relative to the track centerlines. The servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
In some cases, the servo tracks defined by the servo sectors will comprise an eccentricity due, for example, to a non-centric alignment of the disk with the spindle motor. The eccentricity of servo tracks is particularly apparent when the servo sectors are written by a media writer prior to clamping the disk to the spindle motor of a disk drive as compared to writing the servo sectors after installing the disk into the disk drive. In certain designs, the eccentricity of the servo tracks is accounted for by cancelling the effect from the position error signal (PES) used to servo the head in response to the servo sectors, thereby defining substantially circular tracks.
If the data tracks are defined too close to one another (track squeeze), the reliability of the written data may decrease due to adjacent track interference. Accordingly, it may be desirable to verify the spacing of the tracks as part of a validation of each production disk drive, or as feedback for optimizing the servo writing process and/or for optimizing the servo controller. A known method for detecting track squeeze is illustrated in FIGS. 2A-2D wherein in FIG. 2A a track squeeze occurs due to a deviation in the sinusoid representing the C servo burst relative to an offset from the centerline of a track (e.g., because the C servo burst was written incorrectly). FIG. 2B shows the resulting perturbation in the position error signal (PES) generated from reading the servo bursts.
The track squeeze may be detected by evaluating the servo bursts at selected offsets from a track centerline. For example, a prior art technique for detecting a track squeeze condition is to generate a track squeeze indicator (TSI) according to:(A−B)2+(C−D)2.
FIG. 2C illustrates the above track squeeze indicator relative to the PES of FIG. 2B and the corresponding track squeeze condition. As shown in FIG. 2C, the track squeeze indicator is more pronounced at offsets of zero and one-half of a track as compared to offsets of one-quarter and three-quarters of a track. This is further illustrated in FIG. 2D which shows a normal magnitude of the track squeeze indicator (solid line) and the deviation of the track squeeze indicator (dashed line) due to the track squeeze condition. As illustrated in FIG. 2D, the deviation is most pronounced along the diagonal lines where (A−B) equals (C−D) which corresponds to the zero offset and one-half track offset shown in FIG. 2C.